The remaining recordable or reproducible amount of a magnetic tape used in a tape magnetic recording/reproducing apparatus such as a VTR (Video Tape Recorder), a camcorder or an AIT (Advanced Intelligent Tape) backup system is generally calculated on the basis of the length of the magnetic tape which is currently wound on either one of a reel from which the magnetic tape is supplied (hereinafter referred to as the supply side reel) and a reel on a side for taking up the magnetic tape supplied (fed) from the supply side reel (hereinafter referred to as the take-up side reel).
However, in a case where the remaining amount of the magnetic tape is calculated in this manner, there is a problem that the calculation result contains an error corresponding to variations in thickness of magnetic tapes, so an accurate remaining amount cannot be detected. For example, if there are variations of 10% in tape thickness, calculations of the remaining tape amounts of 60-min tapes will contain an error of a maximum of 6 minutes which is 10% of 60 minutes.